1. Field of the Invention
The present invention relates to monitoring contamination within fluids, and particularly to monitoring molecular and particulate contamination within a flowing fluid.
2. Statement of the Problem
Surface molecular contamination (“SMC”) is a phenomenon known in a wide variety of manufacturing fields. SMC is a chemical contamination of a product or component or other surface by gas-phase molecules of any of a variety of contaminants within a fluid contacting the surface. The fluid having the molecular contamination may be a gas or liquid, and is typically the ambient fluid during one or more processing or manufacturing steps. Those skilled in the art distinguish molecular contamination (“MC”) from particulate contamination, and we shall make this distinction herein. The difference may be expressed phenomenologically in that particulate contamination does not bond with a surface. While particulate contamination may attach itself to a surface due to static electric charge, when the charge is neutralized, it is easily removed such as by a pressurized stream of gas or by washing with deionized water. On the other hand, molecular contamination is not so easily removed because it can chemically bond with a surface. The difference may also be expressed quantitatively in that particulate contamination is generally on the order of 10 nanometers or greater in size, while molecular contamination is generally on the order of a few nanometers or less. This line is not black and white because, generally, in the field, if a few molecules or more are bound together, the contamination is referred to as “particulate contamination”, and the contaminants are referred to as “particles”, and the absolute size depends on the size of the molecules. Contamination by molecular contaminants is distinguished from contamination by particles in the art, and will be so distinguished herein. Particles are detected and counted using particle counters, which employ significantly different technologies than MC monitoring. MC molecules have a typical diameter of, for example, 1 nanometer, while a typical particle diameter is 100 nanometers. Surface molecular contamination is sometimes referred to as airborne molecular contamination (AMC) when the perspective is that of the molecular contaminants being present in ambient air, for example in cleanroom air. In this disclosure, SMC includes AMC.
Typical fluid MC compounds are within SEMI F21-95 Classes A, B, C and D. Some, though, are of no particular class. Detrimental effects of SMC occurring due to MC include, for example, changes in the chemical, electrical, and optical qualities of the surface. These manifest in the end product as a decrease in its performance and reliability, and an increase in its cost due to factors including, but not limited to, the lower percentage that pass manufacturing inspections. For example, in the processing and fabrication of a semiconductor, SMC has several detrimental effects. These effects include T-topping of the resist, defective epitaxial growth, unintentional doping, uneven oxide growth, changes in surface properties, corrosion and decreased metal pad adhesion. Many of these are becoming particularly detrimental as line widths less than 0.18 microns are being used. In the optic industry, SMC is a well-known cause of hazing of optical surfaces. SMC also causes friction problems in certain mechanical devices, as SMC contaminated surfaces may have a significantly higher coefficient of friction than uncontaminated surfaces. SMC also affects the manufacture of hard disk drives and flat panel displays which, for reasons known in the art, are typically carried out in a plurality of “mini” clean rooms.
Sources for MC include inadequate filtration of recirculated air, cross-process chemical contamination, outgassing of cleanroom materials, such as filters, gel sealants and construction materials, as well as contaminants carried in and exhuded by human beings. When the fluid is outdoor “make-up” air, the sources of MC include automobile exhaust, evapotranspiration from plants, and various industrial emissions, and the many chemical compounds and vapors resulting from chemical breakdown of, and interaction between, the molecules within the primary sources.
Other sources of AMC/SMC include cross-process chemical contamination within a bay or across a facility, and recirculated air with inadequate ventilation. Still other sources include outgassing by cleanroom materials, such as filters, gel sealants, and construction materials, especially new fabrics, and various contaminants emanating from industrial equipment, such as pumps, motors, robots and containers. Another source is accidents, including chemical spills, and upsets in temperature and humidity controls. Still another source is people, including their bodies, clothes, and their personal care products.
One behavioral characteristic by which SMC is classified is whether it is “reversible” or “irreversible”. Reversible SMC increases, or accumulates, when MC exists in a surface's ambient environment, but decreases or evaporates when the surface is no longer exposed to the causative MC. Reversible SMC arises, for example, from MC chemicals with low boiling points, or from contaminant chemicals that do not chemically react with or bond to the surface. Surfaces exposed to such MC types typically re-equilibrate quickly to the MC chemical density within the ambient fluid. Using water as an example MC, dew would be an example of a reversible SMC.
Irreversible SMC, on the other hand, remains on the surface even after the MC is lowered. An example of irreversible SMC is the haze that typically forms on the interior window surface of a new automobile, due to outgassing from the new plastic components in the automobile's interior. The irreversibility is due, generally, to the MC chemical being reactive with the surface and/or the MC chemical having a very high boiling point, (e.g., greater than 150° C.).
FIG. 1 is an example plot of SMC on a measurement surface near a photoresist tool used in the fabrication of an integrated circuit on a semiconductor wafer. The horizontal axis represents time, with the units being days, and the vertical axis represents the SMC density in nanograms per meter squared. The FIG. 1 example shows a first reversible SMC, labeled HV, which occurs as a result of MC arising from high volatility mass deposition events. Due to the high volatility of the MC, the SMC resulting from these events manifests as spikes. FIG. 1 also shows a second reversible SMC, labeled MV, occurring from MC causing medium volatility mass deposition. As shown, the time decay of the MV kind of SMC is slower than that of the high volatility HV SMC. Also shown in FIG. 1 is the steadily increasing level of irreversible SMC, labeled as the trend line LV, arising from MC chemicals having either low volatility or a bonding to the measurement surface.
As seen from FIG. 1, SMC frequently arises from multiple and simultaneous causes. For example, tool maintenance performed near the photoresist tool is an event causing reversible SMC spikes. Another cause is a tool chemical refill operation. The causation is determined in significant part by correlating the time of an SMC event, which is shown by the horizontal axis of the graph, with the time of an activity, such as the above-identified tool maintenance. For this reason, real-time SMC measurements can be very helpful in identifying events that create SMC.
In view of the above-described sources, types and causes of MC and SMC, as well as their effects, many requirements are placed on an MC/SMC monitoring system. These requirements include sensor reliability, chemical selectivity, sensitivity, accuracy in representing MC levels, and the time delay between an MC event and the time that an alert as to the change in the MC/SMC level is generated.
Still another significant requirement is for monitoring of MC within a flowing gas or liquid. A flow having pure fluids is frequently required in manufacturing processes.
There are a number of MC and SMC monitoring systems known in the art, meeting one or more of the above-identified objectives and requirements, but all have significant shortcomings. These include cost, complexity and time requirements for conducting the tests, susceptibility to human error, limitations as to detectable MC species, and poor time resolution. In addition, existing systems cannot typically monitor fluid within a continuous flow. Instead, the existing systems typically monitor a reservoir tank from which the flow originates. Further, many of the existing devices and methods for measuring MC, such as the test wafer method, have a time delay of up to several days. Although this may be tolerable for some applications, there are others where such a time delay is not acceptable.
One known system for measuring MC is the “sorption tube sampling and gas chromatography/mass spectroscopy” method. This system is referenced herein as “GC/MS.” For measuring MC in, for example, air, a GC/MS monitoring system pulls a large sample of the air through a sorption tube, which preconcentrates the contaminants. The sorption tube is then thermally desorbed, and the sample flushed into the GC/MS system. The sorption tube, or GC/MS system, may be adequate for detecting some low and mid boiling point organic compounds, typically provides some selectivity as to which MC chemical is to be detected, and has relatively high sensitivity. However, GC/MS is not real-time, meaning that it cannot provide MC readings immediately after a causative event. GC/MS is also time-consuming, involves complex operations, and is susceptible to chemical reactions occurring in the sample apparatus. GC/MS is also typically inadequate at detecting inorganic MC chemicals, and at detecting MC chemicals having a high boiling point.
Other known systems for measuring MC are a “bubbler sampling and impinger ion chromatography” system, referenced herein as “IC”, and “atomic absorption spectroscopy,” which is referenced herein as “AAS”. Typical measurement of, for example, air using AAS or IC begins by pulling a large sample of the air through a liquid bubbler. This obtains, within the liquid, a preconcentration of the air's MC. The liquid is then injected into the IC or AAS system, which detects certain classes of inorganic MC molecules. The AAS and IC methods have good sensitivity and selectivity as to which MC is to be detected. However, like the sorption tube method, the AAS and IC methods are time consuming and do not provide real-time measurements. The AAS and IC methods are also susceptible to chemical reactions occurring in the sample apparatus. Further, they are typically not very good at detecting organic MC chemicals.
A Still another known MC measurement system is the “ion mobility spectrometer,” referenced herein as “IMS”. In a typical measurement of, for example, air using IMS the air is pulled over a membrane that passes only certain chemicals. The chemicals that pass through the membrane are then ionized by nickel 63. The ions are then separated by their mobility in an electric field.
Chemiluminescence is another known MC measurement system. A typical chemiluminescence system employs ozone within a monitor, which reacts with ammonia/amines to form unstable intermediate molecules. The intermediate molecules decay and, in doing so, generate light which is detected. Chemiluminescence is a real-time MC monitoring technology, with good selectivity and sensitivity. However, it detects only ammonia and amines, and it carries problems in relating the MC data to a particular SMC condition.
Each of the above-identified systems directly measure MC suspended within a sample of the subject fluid. Another class of MC measurement systems and technologies measures surface molecular contamination, or SMC, on a surface exposed to the fluid-born MC.
One known SMC measurement system is the test wafer method, which exposes a test wafer to a fluid for an extended period, typically ranging from three to seven days, removes the wafer and measures it by thermal desorption GC/MS analysis, or by time of flight secondary ion mass spectroscopy, or TOF/SIMS.
The test wafer method solves some of the problems listed above, particularly the scope of MC chemicals that it detects. Basically, the test wafer method can, theoretically anyway, detect anything that stays on the test surface. However, it is not real time, and it has very poor resolution in correlating the time of an MC event to the detected SMC. The test GC/MS analysis method is also inherently deficient in detecting MC species that react with the test surface. It also has shortcomings in detecting MC species having low boiling points, because the SMC for such species is reversible and, therefore, evaporates before the wafer can be tested. Also, if TDGC/MS detection is used to analyze the test wafer, inorganic compounds are not adequately detected.
Another known SMC measurement system is the quartz crystal microbalance system. This system employs a bulk piezoelectric crystal in an oscillating circuit, where the frequency of oscillation changes upon contaminants adsorbing on the crystal surface. In addition, the crystal surface mimics some product surfaces, whereby crystal surface contamination can be monitored overtime using TOF/SIMS or a test wafer to identify a contaminant. However, the sensitivity of the quartz crystal microbalance system, in terms of the frequency change caused by contaminants, is low. Therefore, it is not adequate for typical SMC detection.
Still another known SMC measurement system is the surface acoustic wave (“SAW”) sensor system. An example SAW sensor is the commercially available “AiM” monitor from Particle Measuring Systems, Inc. The AiM SAW-based monitor contains two SAW crystals, one having an exposed surface and the other being hermetically sealed. The exposed SAW crystal surface interacts with most MCs of interest in the same way as the surface for which exposure to MCs is of concern. Each SAW is within a resonant circuit, the circuit having a resonant frequency determined, in part, by characteristics of the SAW. SMC on the SAW crystal surface changes its characteristics which, in turn, change the resonant frequency. By comparing the resonant frequency of the circuit having the exposed SAW with the circuit having the sealed SAW, a signal reflecting the amount of SMC on the SAW surface is obtained.
A SAW sensor system such as the AiM monitor satisfies some, but not all, of the objectives required of an MC/SMC monitor system for current and projected industrial needs. For example, the AiM monitor has good sensitivity, and provides real-time SMC data, because the SAW frequency changes as SMC accumulates on its surface. However, the AiM monitor cannot measure MC within a continuous fluid flow.